Field of the Invention
The present invention relates to a novel blister-free pressure-sensitive adhesive sheet. More particularly, the present invention relates to a pressure-sensitive adhesive sheet which can be applied and bonded to the surface of shaped articles of various kinds of plastic resins without occurrence of blisters.
Background Information
As is well known, pressure-sensitive adhesive sheets having a coating layer of a pressure-sensitive adhesive are widely used and are now indispensable in a variety of industrial fields, for example, as a printed label, seal on wrapped packages and the like. This is because pressure-sensitive adhesive sheets can be readily bonded to the surface of various substrates conveniently by merely putting on and lightly pressing to the substrate surface and, different from adhesives in general, the pressure-sensitive adhesive forming the coating layer of the sheet contains no organic solvent so that advantageously no problems of environmental pollution are caused thereby.
Pressure-sensitive adhesive sheets can be applied and bonded to the surface of a substrate article of any material almost without limitation including paper, wood, metals, ceramics, plastics and the like. As a consequence of the trend in recent years that light-weight articles are preferred in general, in particular, the quantity of plastic-made products on the way of distribution is rapidly increasing year by year so that the occasion, in which pressure-sensitive adhesive sheets are attached to plastic-made goods or plastic-made packages of goods as a label, is also increasing rapidly. When a transparent pressure-sensitive adhesive sheet used in such an application has a printed pattern as desired, the plastic-made article to which such a sheet is attached on the surface may have an outer appearance as if it were printed directly thereon.
Apart from the application as a label or seal of packages, pressure-sensitive adhesive sheets are sometimes attached to the surface of a board of polycarbonate resins used as a window pane by virtue of the high transparency with an object to protect the surface from scratches or to control transmission of sun light therethrough.
Pressure-sensitive adhesive sheets in general have a serious problem that, when an adhesive sheet is applied and bonded to the surface of an article made of a polystyrene resin, polycarbonate resin and the like, so-called blisters are formed in many instances due to incomplete squeeze-out of the air between the substrate surface and adhesive sheet attached as a label or a protecting sheet. This problem is unavoidable when the face-stock film of the pressure-sensitive adhesive sheet is made of a plastic resin having a gas barrier property such as conventional polyester resins.
When blisters are caused in the pressure-sensitive adhesive sheet attached to the surface of an article as a label, not only the article has a very unsightly appearance which decreases the commercial value but also the label and the like attached to the surface are liable to eventual falling. In a printed label made of a see-through pressure-sensitive adhesive sheet, in particular, it is unavoidable that the appearance is extremely poor because the layer of the adhesive coating exhibits a pockmarked or white and cloudy appearance through the face-stock film. The mechanism of this undesirable phenomenon is that the plastic film as the face-stock film of the pressure-sensitive adhesive sheet acts as a barrier against escape of the gas in a small volume occurring between the face-stock film and adhesive coating layer so that the gas retained there forms a number of gas-filled blisters.
One of the possible means to solve this problem by dissipating the gas in the blisters is to provide a gas-permeable layer made of paper, porous polyurethane and the like between the face-stock film and the adhesive coating layer of the pressure-sensitive adhesive sheet. This method, however, has no general applicability because the overall thickness of the adhesive sheet is necessarily increased so much and the adhesive sheet can no longer be used as a see-through label due to the loss of transparency by the interposed gas-permeable layer.
It is also known that the problem due to the formation of blisters in a pressure-sensitive adhesive sheet can be considerably decreased when a pressure-sensitive adhesive of a certain type is used for the adhesive coating layer. Examples of such a pressure-sensitive adhesive hitherto proposed include those rubber-based ones comprising a block copolymer of styrene-isoprene-styrene type or a block copolymer of styrene-butadiene-styrene type compounded in a specific formulation. Pressure-sensitive adhesive sheets having an adhesive coating layer of such a rubber-based adhesive are, however, not quite satisfactory because of the poor weatherability of the adhesive layer inherent in rubber-based pressure-sensitive adhesives.